Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats

Appendix  VI.                          1177

two sets of guides n n are fixed to the chamber. The pieces «a n^
are of short length, existing on one sid^ of wheel only, but
sufficient to take the whole flow from the *five valves q q. The
first set of guides pp are placed very acutely, and have long
passages, corresponding in fact to de Laval nozzles, and supplying
velocity energy. The steam enters at c, and passes through
wheel b to the intermediate chamber d, whence it traverses the
blades of the second wheel a as it did the first, exhausting at e e.
The governor / actuates levers k /, and by suitable rods rotates a
cam shaft which opens or closes the valves q q in succession, so
that nozzles pp are wholly filled or empty, thus increasing the
efficiency by cut off instead of throttling. These turbines are
built of 15 to 5000 H.P., and weigh -Jth of a reciprocating engine
of like power. The decrease of steam weight used when super-
heating is i % for every 5 % rise, as already stated, while in
one case at least the equivalent of i lb. of coal per LH.P. was
expended at f to full load, or equal to that of good Sulzer engines.
The general consumption of steam is 16 Ibs. per B.H.P. hr., the
boiler steam of 118 Ibs. pressure being reduced to 114 Ibs. at

Pressure Turbines.—The Parsons turbine has already been
described at pp. 895 and 966 in its two forms of radial and
parallel flow. As stated, the former was only adopted on account
of patent difficulties, and, when these disappeared, the latter was
returned to. The parallel flow turbine is, therefore, the only one
to be considered, though the same principle of pressure and slow
•expansion is adopted in both. The turbine is shewn in section
at p. 966, and a few more points will be explained by reference to
Fig. 1034. The governor here shewn is that of a Brown-Boveri
Parsons turbine. Steam enters at s, and flows through main
valve K, which is lifted by the steam pressure beneath piston L,
when valve H is closed. On the governor spindle is an'eccentric
•c, which operates a bell-crank C to regularly raise and depress
valve H, the lever H vibrating rigidly. When the latter is opened,
the piston j is put in equilibrium, permitting spring L to close the
main valve, and, conversely, when H is closed, K opens. The
governor sleeve D is connected by levers E and F to the valve H,
•which it endeavours to keep raised at some certain average height,